LoginActivity

package com.demo.parking.ui;

import android.content.DialogInterface;
import android.content.Intent;
import android.os.Bundle;
import android.support.v7.app.AlertDialog;
import android.support.v7.app.AppCompatActivity;
import android.view.View;
import android.widget.Button;
import android.widget.EditText;
import android.widget.Toast;

import com.demo.parking.ApiHelper;
import com.demo.parking.Constants;
import com.demo.parking.R;
import com.demo.parking.model.User;
import com.demo.parking.util.PrefsUtil;

import org.json.JSONException;
import org.json.JSONObject;

import java.io.IOException;

import okhttp3.ResponseBody;
import retrofit2.Call;
import retrofit2.Callback;
import retrofit2.Response;
import retrofit2.Retrofit;
import retrofit2.converter.gson.GsonConverterFactory;

public class LoginActivity extends AppCompatActivity {

    private EditText inputEmail;
    private EditText inputPassword;
    private Button btnLogin;
    private ProgressBarWidget progressBarWidget;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_login);

        inputEmail = (EditText) findViewById(R.id.inputEmail);
        inputPassword = (EditText) findViewById(R.id.inputPassword);
        btnLogin = (Button) findViewById(R.id.btnLogin);
        progressBarWidget = new ProgressBarWidget(LoginActivity.this);

        btnLogin.setOnClickListener(new View.OnClickListener() {

            @Override
            public void onClick(View v) {
                if (validLoginEntries()) {
                    login(inputEmail.getText().toString(), inputPassword.getText().toString());
                }
            }
        });
    }

    public void gotoRegistration(View v) {
        Intent i = new Intent(this, RegistrationActivity.class);
        startActivity(i);
        finish();
    }

    public boolean validLoginEntries() {
        boolean valid = true;

        String email = inputEmail.getText().toString();
        String password = inputPassword.getText().toString();

        if (email.isEmpty() || !android.util.Patterns.EMAIL_ADDRESS.matcher(email).matches()) {
            inputEmail.setError(getString(R.string.text_invalid_email));
            valid = false;
        } else {
            inputEmail.setError(null);
        }

        if (password.isEmpty() || password.length() < 4 || password.length() > 10) {
            inputPassword.setError(getString(R.string.text_invalid_password));
            valid = false;
        } else {
            inputPassword.setError(null);
        }

        return valid;
    }

    private void login(String email, String password) {
        // show progress bar
        progressBarWidget.show();
        Retrofit retrofit = new Retrofit.Builder()
                .baseUrl(Constants.API_BASE_URL)
                .addConverterFactory(GsonConverterFactory.create())
                .build();

        ApiHelper helper = retrofit.create(ApiHelper.class);
        Call<ResponseBody> call = helper.login(email, password);

        call.enqueue(new Callback<ResponseBody>() {
            @Override
            public void onResponse(Call<ResponseBody> call, Response<ResponseBody> response) {
                try {
                    final String body = response.body().string();
                    JSONObject loginResponse = new JSONObject(body);
                    if (loginResponse.has("success") && loginResponse.getInt("success") == 200) {

                        // create user object
                        User user = new User();
                        user.setSessionID(loginResponse.getString("session_id"));
                        user.setUsername(loginResponse.getString("username"));

                        // save session_id into SharedPreferences
                        PrefsUtil.getInstance(getApplicationContext()).setSessionID(user.getSessionID());

                        // show toast
                        Toast.makeText(getApplicationContext(), R.string.text_success_alert_login_message, Toast.LENGTH_LONG);

                        // redirect back to main screen
                        Intent i = new Intent(getApplicationContext(), MainActivity.class);
                        Bundle bundle = new Bundle();
                        bundle.putParcelable(Constants.BUNDLE_USER, user);
                        i.putExtras(bundle);
                        startActivity(i);
                        finish();
                    } else {
                        showErrorDialog(getString(R.string.text_error_alert_title), loginResponse.getString("message"));
                    }
                } catch (IOException e) {
                    e.printStackTrace();
                } catch (JSONException e) {
                    e.printStackTrace();
                }
            }

            @Override
            public void onFailure(Call<ResponseBody> call, Throwable t) {
                t.printStackTrace();
                showErrorDialog(getString(R.string.text_error_alert_title), getString(R.string.text_error_alert_login_message));
            }
        });
    }

    private void showErrorDialog(String title, String message) {
        // hide progress bar
        progressBarWidget.hide();

        AlertDialog.Builder dialog = new AlertDialog.Builder(LoginActivity.this);
        dialog.setCancelable(false);
        dialog.setTitle(title);
        dialog.setMessage(message);
        dialog.setPositiveButton(getString(R.string.button_ok), new DialogInterface.OnClickListener() {
            @Override
            public void onClick(DialogInterface dialog, int id) {
                dialog.dismiss();
            }
        });

        final AlertDialog alert = dialog.create();
        alert.show();
    }

}